How to make a thick, smooth metal coil using plane extruded along spiral?
I was able to create a spiral as you see here by using the spiral addon.


Note that I used 80 steps.  This is where you get a smooth spiral without using edge split or the subdivision surface modifier.


The bevel profile object was a nurbs circle that I subdivided and changed into a square.  Then I stretched out the square to give me the rectangular shape I wanted.  I can give you more details about how I used subdivide on the nurbs circle to make it into the rectangle if you want.